Method and apparatus for molding indicia in roto-molded plastic containers

ABSTRACT

Disclosed is a method and apparatus for forming indicia on the outside of a roto-molded plastic container during the roto-molding process wherein a template bearing a mirror image of the indicia is removably affixed to the interior wall surface of a mold.

BACKGROUND OF THE INVENTION

The present invention relates to the roto-molding of plastic containersand more specifically to a method and apparatus for roto-molding whereina message or other indicia can be molded directly into the outer surfaceof the plastic container.

Roto-molding is a well known process for manufacturing plasticcontainers. Briefly, in roto-molding a charge of powdered thermoformingresin, such as polyethylene, polypropylene or the like is placed into ahollow mold. The mold is then heated to temperatures which may range upto 700° F. depending upon the resin material. The mold is simultaneouslyrotated about two axes while it is being heated. This distributes thepowdered resin over the entire interior surface of the mold and fusesthe resin into a seamless homogeneous structure with a uniform wallthickness. The mold is then cooled, opened and the plastic containerstripped from the interior of the mold.

Heretofore, when it was desired to form any indicia, such as liquidlevel lines, company names, or other appropriate messages into the sideof the roto-molded plastic container, it was necessary to physicallyengrave a mirror image of the indicia into the wall of the mold. Thus,considerable time, effort and expense was required to change the mold inthis fashion and consequently, the roto-molding of containers withindicia or other markings molded directly into the exterior of thecontainer is seldom, if ever, undertaken.

The use of removable templates temporarily fixed to the interior surfaceof the roto-mold likewise has not heretofore proven successful. This isdue in large measure to the problem of preventing the template frombecoming detached from the side wall of the mold upon exposure forrelatively long durations to the high temperatures involved inroto-molding, sometimes approaching 700° F. for periods of 10-15 minutesor more. Further, the air present in the space between the template andthe side wall of the mold during the heating cycle expands and causesdefects in the surface of the molded article on cooling.

The present invention, however, does provide a method and apparatus formodifying a roto-mold so as to permit the molding of indicia into theside of the roto-molded plastic container. In this respect, theinvention involves in one aspect, the use of a metal template which hasengraved on its surface a mirror image of the indicia to be molded intothe side of the plastic container. This metal template is mechanicallyfastened to the interior surface of the roto-mold by screws, bolts orthe like with a RTV (Room Temperature Vulcanizing) silicone rubber usedas a caulking material to prevent the formation of bubbles in theplastic container due to air entering from around the template.

In another embodiment, the template itself, is cast in a silicone RTVrubber, the template then being attached to the mold using additionalRTV silicone rubber as the adhesive agent. This latter method ispreferred in that it does not destroy the integrity of the mold ascompared to the mechanical technique defined above.

It is therefore the primary object of the invention to provide a methodand apparatus for roto-molding messages into the outer surface of aplastic container without the need for engraving the indicia into thewall of the mold.

The invention will be better understood and additional objects thereofwill become more apparent from the accompanying specification and claimsconsidered together with the accompanying drawing.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a roto-mold showing a template attachedto the wall thereof in accordance with the present invention;

FIG. 2 is a sectional view on an enlarged scale of a portion of theroto-mold wall having mechanically fixed thereto a metal template inaccordance with the present invention;

FIG. 3 is a sectional view of another embodiment of the inventionwherein the RTV Silicone Rubber template is cast in situ on the interiorsurface of the mold wall; and

FIG. 4 is a view similar to FIG. 2 showing a silicone rubber templatefixed to the wall of the roto-mold by an adhesive.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring the the drawings, FIG. 1 shows a roto-mold generally indicatedat 10. The process of roto-molding plastic containers in well-known inthe art. Briefly, the roto-mold consists of a metal shell 12, usuallycylindrical having a smooth interior surface 14. The mold has an opening16 at one end which can be sealed with a metal cover (not shown). Inroto-molding a charge of thermoforming resin powder is placed into themold and the mold is heated and rotated about two axes. This causes thepowder to spread over the entire interior surface of the mold and fuseinto a seamless homogeneous structure with a uniform wall thickness.After cooling the fused plastic article is stripped from the mold andanother resin charge added.

In order to mold indicia into the plastic article, the smooth moldinterior surface 14 must be provided with a mirror image of the indicia.Heretofore, one way of accomplishing this was to engrave the indiciadirectly into the surface of the mold. This, however, was not anacceptable practice as it required considerable time, effort and expensein order to change the indicia once it has been engraved directly intothe mold surface. The present invention, however, eliminates the needfor engraving the mold surface by providing for the attachment of aremovable template 18 to the interior surface of the mold.

Heretofore, removable templates have not been widely used inroto-molding because of the problems involved in maintaining thetemplate affixed to the mold during extended periods of successive usesof the mold. This is due to the relatively high temperatures to whichthe mold is exposed for extended periods of time that may run, forexample, up to 700° F. for 10 to 15 minutes.

The present invention, however, provides embodiments wherein a template18 fixed to the smooth interior surface 14 of the mold can easily beremoved after molding.

In one embodiment as shown in FIG. 2, two or more holes 20 are drilledthrough a section of the mold wall 12. Attached to this section of thewall is a template 22 made of a metal, preferably aluminum, and engravedwith the mirror image of the indicia to be molded into the wall of theplastic container. Welded to the rear side of template 22 are two ormore studs 24 corresponding to the drilled holes 20. Thus, the metaltemplate can be attached to the mold wall simply by inserting studsthrough holes 20 and threading on nuts 26. A layer 28 of RTV-silicone 60Rubber (Regd. TM of General Electric Co.) is then placed between metaltemplate 22 and mold wall 12 along the periphery of template 22. Thislayer 28 placed in the liquid state will cure to form a resilient masswhich seals completely the area beneath the template 22. This sealprevents any air transfer, which might otherwise occur during theheating and cooling cycles from air pockets which may be trapped beneaththe template. If the air transfer occurs it is likely to cause bubblesor other defects in the molded product adjacent to the area of thetemplate.

In another embodiment of the invention, as shown in FIG. 3, the templatematerial is cast in place directly on the smooth interior surface 14 ofthe mold wall 12. In this embodiment a small enclosed dam 30 of modelingclay or the like is built up on the smooth surface 14 of the mold. Aliquid RTV silicone indicated at 32 is poured into the space surroundedby the dam. An impression plate 34 is then pressed down onto the liquidRTV silicone and kept there until the silicone has solidified. Theimpression plate 34 is then removed, leaving a mirror image of theimpression on the surface of the silicone. The molding clay dam 30 isthen removed and the edges of the silicone are trimmed. The cast RTVsilicone rubber on curing adheres to the inside wall of the mold. Afterbeing used for molding containers the formed template 32 is then peeledfrom the smooth surface 14 of the mold.

In still another embodiment of the invention, as shown in FIG. 4, atemplate 40 of either RTV or heat cured silicone is made outside of theroto-mold and then attached to the smooth inner surface 14 of theroto-mold using a layer 42 of RTV 60 silicone rubber (Regd. TM ofGeneral Electric Co.) as an adhesive.

Making the silicone template 40 from an RTV silicone is a relativelysimple matter and involves only the fabrication of a suitable matrixboard and then pouring the liquid RTV silicone into the matrix board.When the silicone is set, it is simply peeled from the matrix board as apad and is a mirror image of the surface of the matrix board. Where thesilicone template 40 is made from a heat cured silicone, a slightly moreinvolved process is involved. In this respect, the heat curable siliconeis placed in the matrix board and subjected to high pressure andtemperature, as for example, pressure of 1,000 lbs per square inch at350° F. These conditions are maintained for approximately 15 minutes tocure the silicone compound. Thereafter, the template is removed from thematrix board and subjected to a post cure heat treatment at 425° F. forabout 3 to 4 hours. If desired, an additional post cure heat treatmentcan be accomplished by subjecting the template to about 550° F. for anadditional 1 to 4 hours. These post cure heat treating cycles arecarried out to insure that there is no shrinkage of the template duringthe actual roto-molding process.

The heat cured silicone template 40 is then affixed to the smoothsurface 14 of the mold with an RTV silicone layer 42 being used as theadhesive. An empty cycle is first performed on the roto-mold wherein themold is heated to approximately 525° F. for 10 to 15 minutes withoutresin. Thereafter, a conventional roto-molding process can be carriedout wherein the powdered resin is introduced directly into the mold asset forth above.

A suitable adhesive used for the template should be heat resistant, havea low coefficient of expansion and be easily strippable from the moldwall, so that after molding one type of indicia on the containers theadhesive is stripped and the template easily removed. The inner wall ofthe mold can then be cleaned and a different template attached. Theremoved template may be preserved to be used again.

An example of such heat resistant, strippable and resilient on curingadhesive is a RTV silicone rubber. One such adhesive is RTV 60 (Reg'd TMof General Electric) silicone rubber which is a dimethyl siloxanecompound cured with dibutyl tin dilaurate or tin octoate.

The template itself could be made out of a variety of materials: metal,a photoresist on a metal, silicone rubber (RTV or heat cured), orfluorocarbons. These are only a few examples of materials suitable astemplates although the invention is not limited to these alone. In orderfor the material to be suitable for templates, it must have thefollowing properties. It should be heat resistant at 750° F. for atleast 15 minutes, be bondable to the inner wall of the mold, beresilient and non-sticking to the resin material being roto-molded.

In the preferred embodiment, the template is made out of a heat curedsilicone rubber and affixed as a pad to the inner wall of the mold withRTV 60 a silicone adhesive (Registered TM of General Electric). Thismethod is preferred in that it does not destroy the integrity of themold due to mechanical damage.

Cured RTV 60 has the following typical properties.

sp. gr.: 1.47

Hardness Shore A: 60

Tensile Strength psi: 800

Elongation %: 130

Tear Resistance die B lb/in.: 40

Linear Shrinkage: 0.2-0.6

Coefficient of Thermal Expansion: 11.4×10⁻⁵

The physical properties of heat cured silicone template are as follows:

Hardness Shore A: 50

Tensile Strength psi: 1000

Elongation %: 400

Tear Resistance die B lb/in: 65

Compressive Strength @

22 hours 350° F.: 23%

70 hours 300° F.: 21%

Though the specification describes molding indicia on the outside of aroto-molded container the process lends itself to molding indicia on theoutside of any roto-molded article.

Thus, it should be appreciated that the invention does provide a methodand apparatus for roto-molding specialized messages or other indiciadirectly into the walls of a roto-molded container and that this isaccomplished without materially altering the integrity of the molditself.

Having thus described the invention in detail, what is claimed as newis:
 1. A method of rotomolding articles having indicia formed in theexterior wall thereof, comprising the steps of:(a) forming a dam on theinterior surface of a rotomold which encloses an area corresponding tothe area on the article to be provided with the indicia; (b) pouring aliquid room temperature vulcanizing silicone into the area surrounded bysaid dam; (c) pressing an impression plate onto the surface of saidliquid room temperature vulcanizing silicone and maintaining theposition of said impression plate until RTV silicone has set;(d)removing said dam and impression plate to leave a template which carriesa mirror-image of the indicia to be molded adhering to the interiorsurface of the rotomold; (e) placing a charge of thermo forming resininto the rotomold and heating the mold externally while rotating saidmold to distribute and fuse said resin over the interior mold surfaceincluding said template thereby forming an article in which the exteriorwall of the article has the mirror-image of said template surface moldeddirectly therein; and thereafter (f) removing the article from said moldwhile retaining said template adhered to the interior mold surface.